The present invention relates to a rubber composition for a tire sidewall and to a pneumatic tire using the same. More specifically, the present invention relates to a rubber composition for a tire sidewall which can improve the rolling resistance property to a large degree without causing a deterioration in crack resistance and appearance, and to a pneumatic tire using the rubber composition for sidewalls.
The sidewall of a pneumatic tire is made of a rubber composition containing natural rubber, isoprene rubber and butadiene rubber as a main rubber component, which have a high degree of unsaturation and a double bond in the main chain. It is known that the double bond part of such high unsaturation rubber has a nature of reacting with ozone to induce depolymerization, and therefore cracks tend to generate on the rubber surface when the rubber is formed into a tire and the tire is left or run.
In order to prevent these problems, a crack inhibitor such as an amine antioxidant or a wax is compounded in the rubber composition of sidewall. However, when these chemicals bloom on the sidewall surface, the amine antioxidant tends to turn brown while the wax is likely to become white, and there has been a problem that the appearance of the sidewall becomes inferior in both cases.
A wide variety of methods are proposed for the purpose of solving the above problem. For example, it is described to apply butyl rubber or EPDM rubber on the surface of sidewall or tread of a tire to improve weatherability and crack resistance (JP-A-9-272306, JP-A-4-362404, JP-A-4-39105, JP-A-3-276801, JP-A-2-102805, JP-A-2-45202 and JP-A-1-207307). However, though these methods contribute to improved appearance of tires, there is the problem that flexing durability and crack growth resistance becomes inferior and adhesion with other rubbers is decreased.
An object of the present invention is to provide a rubber composition for a tire sidewall capable of preventing color change to brown, which has been regarded as a problem of automobile tires, while achieving sufficient ozone resistance and low heat build-up characteristics, and a pneumatic tire having a sidewall comprising the rubber composition.
In the present invention, rolling resistance property (low heat build-up characteristics) is improved without reducing color change resistance and ozone resistance by compounding calcium carbonate to the rubber composition for sidewall.
That is, the present invention relates to a rubber composition for sidewall comprising, 5 to 150 parts by weight of carbon black having nitrogen adsorption specific surface area of 70 to 300 m2/g and 5 to 100 parts by weight of calcium carbonate based on 100 parts by weight of a rubber component comprising 20 to 70% by weight of a natural rubber and 30 to 80% by weight of a synthetic diene rubber.
It is preferable that the calcium carbonate has nitrogen adsorption specific surface area of at least 5 m2/g.
The present invention also relates to a pneumatic tire having sidewalls comprising the rubber composition.